1. Matching Relationship Between Operating Speed and Magnetic Field

• Synchronous motors: The rotor speed is always completely consistent with the speed of the stator rotating magnetic field, which is called “synchronous operation”. Their rotors either have built-in permanent magnets or generate a fixed magnetic field by passing direct current through the excitation winding. After the stator rotating magnetic field is formed, it will pull the rotor to rotate synchronously like “a magnet attracting iron”, with no speed deviation.
• Asynchronous motors: The rotor speed is always lower than the speed of the stator rotating magnetic field, resulting in a “speed difference” (this is the origin of the name “asynchronous”). Their rotors do not have an independent magnetic field; instead, they rely on the stator magnetic field to cut the rotor conductors to generate an induced current, which in turn forms a rotor magnetic field. Only when the rotor speed is slower than the stator magnetic field can the continuous cutting of conductors by the magnetic field be ensured, maintaining the induced current and rotor rotation. Therefore, the speed difference is a necessary condition for the operation of asynchronous motors.

2. Starting Performance and Torque Characteristics

• Synchronous motors: They have the problem of “difficult starting”. Since the rotor magnetic field is fixed, the speed of the stator rotating magnetic field is extremely high at the moment of starting, and the rotor cannot keep up immediately due to inertia, easily causing “loss of synchronization” (i.e., the rotor cannot be pulled to rotate by the magnetic field). Therefore, they cannot be started by directly energizing. Usually, auxiliary devices (such as a small asynchronous starting winding) are needed to first rotate the rotor to a speed close to the synchronous speed, and then excite current is applied to complete “pull-in synchronization”. Additionally, their starting torque is small, making it difficult to drive heavy loads for starting.
• Asynchronous motors: They are easy to start and have more flexible torque characteristics. No auxiliary devices are required; they can be started by directly energizing. During the starting process, the rotor speed gradually increases, and the speed difference gradually decreases. According to the different rotor structures, asynchronous motors can be divided into squirrel-cage type and wound-rotor type: Squirrel-cage motors have moderate starting torque and are suitable for light-load scenarios (such as fans); Wound-rotor motors can increase the starting torque by connecting resistors in series in the rotor circuit, which can meet the starting requirements of heavy loads (such as cranes).

3. Efficiency and Power Factor Adjustment Capability

• Synchronous motors: They have higher efficiency and can adjust the power factor. Since the speed is always synchronous, there is no “slip loss” (one of the main losses of asynchronous motors) caused by the speed difference. Less energy is wasted during long-term operation, and the efficiency advantage is more obvious in high-capacity equipment (such as large generators and industrial compressors). In addition, the power factor of synchronous motors can be controlled by adjusting the excitation current. When the excitation current is sufficient, the motor can output reactive power to the power grid, improving the power factor of the power grid (asynchronous motors cannot do this). Therefore, they are often used as “synchronous condensers” to stabilize the power grid voltage.
• Asynchronous motors: They have relatively lower efficiency and fixed power factor. Due to the existence of slip loss, especially during light-load operation, the efficiency will decrease significantly (for example, the efficiency is close to zero when no-load). At the same time, their power factor is always lagging (i.e., they need to absorb reactive power from the power grid to establish a magnetic field) and cannot be actively adjusted. Large-scale use may lead to a decrease in the power grid power factor and an increase in power grid losses.

4. Differences in Application Scenarios

• Synchronous motors: They are more suitable for scenarios with high requirements for speed accuracy, efficiency, and power grid stability:
  1. Power generation field: All large generators (such as thermal and hydroelectric generators) are synchronous motors, because they can ensure stable speed and output electrical energy with a constant frequency (the frequency of China’s power grid is fixed at 50Hz, which needs to be realized by relying on synchronous motors).
  2. Industrial heavy-load equipment: Large industrial compressors, water pumps, ball mills, etc., use their high efficiency and stable speed to reduce long-term operating costs.
  3. Power grid regulation: Used as synchronous condensers to improve the power grid power factor and alleviate the problem of insufficient reactive power in the power grid.
• Asynchronous motors: Due to their simple structure, low cost, and convenient maintenance, they have become the mainstream choice in civil and small-to-medium-sized industrial scenarios:
  1. Civil equipment: Household appliances (such as air conditioners, washing machines, electric fans) and small water pumps all use squirrel-cage asynchronous motors to meet daily light-load needs.
  2. Small-to-medium-sized industrial equipment: Machine tool spindles, conveyor belts, blowers, etc., do not require extremely high accuracy and efficiency, so the cost-effectiveness advantage of asynchronous motors is more prominent.
  3. Heavy-load starting scenarios: Wound-rotor asynchronous motors are used in equipment such as cranes and hoists, where the starting torque is adjusted by modifying the rotor resistance.